1. Field of the Invention
The present invention relates, in general, to optical structures that enable optical beam transformation between a large-mode-size waveguide and a small-mode-size waveguide, and methods of making the same. In particular, the present invention relates to methods for transforming the optical mode between a photonic device and one or more optical fibers. The present invention also relates, in particular, to the integrated fabrication of such structures on a module platform or the photonic device, their connections with one or more input/output optical fibers.
2. Description of the Related Art
The current strong demand for bandwidth over the Internet has resulted in great demand for photonic device components in optical communications and data or information processing. These device components include fiber optics, non-linear crystal optics, and integrated optics in such material systems as dielectrics, polymers, optical crystals, and semiconductors (also called electro-optic or optoelectronic systems).
Optical-crystal and dielectric-material-based discrete optical components such as LiNbO3-based-modulators, glass ion-exchange-based optical power splitters and flame-hydrolysis-deposited silica-on-silicon multiplexers/demultiplexers, can play certain roles, but their sizes are generally large and their functions are limited. Hence, in the long run, it is very unlikely that they can compete with waveguide based photonic devices such as Photonic Integrated Circuits (PICs), which can be made very small and also multifunctional with high packing density similar to today's large scale integration of microelectronic circuits.
Waveguides are used for inputting and out putting light energy for such photonic devices to optical fibers. The input/output waveguides in a photonic device are typically made up of dielectric or semiconductor materials. A photonic device may contain one or more such input/output waveguides. Unless otherwise specified, such input/output optical waveguides will be referred to as device waveguides below.
In spite of the promise of waveguide based photonic devices, in general, and photonic integrated circuits, in particular, however, several challenges remain. One challenge is at the optical interface. Light must be efficiently coupled, with high precision and stability, between drastically dissimilar components and materials, in a cost effective, manufacturable way. There are several issues that need to be addressed with respect to this challenge, including the following:                (1) The drastically different spot or mode profile in terms of size and symmetry between a fiber and a photonic device waveguide.        (2) The difficulty in the alignment of a fiber and a photonic device waveguide, as well as of any other intermediate component such as a ball lens.        (3) The difficulty of coupling multiple fibers to a photonic device with multiple device waveguides efficiently in a cost effective way.        
Prior-art efforts addressing each of these challenges are summarized below.
(1) Prior Art in Dealing with the Mode-Size Conversion Issue
With regard to mode-size conversion, in order to ensure single-mode operation (as required for high-speed, large-capacity optical signal manipulation), the dimension of a device waveguide is typically one order of magnitude less than that of a silica fiber waveguide. The result is a substantial mode-field mismatch between these two waveguides. As shown in FIGS. 1(a) and (b), for optimal performance, the mode profile of a single-mode optical fiber 110 is circular, and its size is generally about 5 to 10 μm in diameter, whereas the mode profile of a photonic device waveguide 120 is elliptical and its dimension is typically less than 1 to 3 μm—as small as 0.2 μm for high-density photonic integrated circuits.
Various methods are currently used for transforming the optical modes between an optical fiber and a device waveguide. These methods are broadly summarized below.
Method 1—Butt-Coupling Method
The simplest coupling arrangement is a direct butt-joining between a fiber and a semiconductor laser (or other semiconductor waveguide) as shown in FIG. 2. Since light is only required to couple in one direction—i.e., from the laser 210 to the fiber 220—one can adjust the gap distance 230 to allow the divergent cone of light 240 to expand and roughly match the size of a fiber core 250. One problem with this approach is the relatively low coupling efficiency caused by the large divergence angle and the fact that a fiber can only capture and guide a narrower cone of light within a small capturing angle. As a result, the typical coupling efficiency for a direct butt-joining is less than 5–30% depending on the size of the device waveguide. In spite of the low coupling efficiency, this technique is being explored by NEC of Japan (among others) for low-cost mass packaging of transceivers because this technique requires the fewest of components, which minimizes component cost. (Kenji Yamauchi et al., “Automated mass production line for optical module using passive alignment technique,” 50th Electronic Components and Technology Conference, May 21–24, 2000, Las Vegas, Nev., USA).
Method 2—Lensed Fiber or Microlens Method
A method improved over the direct butt-joining technique is to make the fiber end into a lens 310 (lensed fiber) as shown in FIG. 3 so that more light can be captured by the fiber. (Kazuhiko Kurata, “Mass production techniques for optical modules,” 48th Electronic Components and Technology Conference, May 27–28, 1998, Seattle, Wash., USA). Another improved method uses a separate lens 410 placed in the gap 420 as shown in FIG. 4. Various lenses have been used, including glass ball lenses and GRIN (graded refractive index) rod lenses, as well as aspheric injection molded plastic lenses. (Keith Anderson, “Design and manufacturability issues of a co-packaged DFB/MZ module,” 49th Electronic Components and Technology Conference, Jun. 2–4, 1999, San Diego, Calif., USA). With these lenses, the coupling efficiency is increased to 50% to 70% for device waveguide mode about 2 μm in size.
Method 3—Cylindrical Lenses Method
Besides geometric discontinuities between a device waveguide and an optical fiber, the imperfect coupling efficiency results also in part from the elliptical shape of the light cone emerging from a typical device waveguide such as that from a Fabry-Perot cavity semiconductor laser, which causes a non-perfect match with the circular mode pattern of the fiber. A method to correct for such elliptical or astigmatic beam shape is shown in FIG. 5, which illustrates the use of a combination of two perpendicular cylindrical lenses 510 and 520 of different focusing powers along the vertical lens (510) and lateral lens (520) directions, which can circularize the elliptical beam and theoretically increase the coupling efficiency to about 85% for a typical semiconductor laser with a mode size of about 1 μm (vertical) by 3 μm (horizontal). (Sun-Yuan Huang et al., “High coupling optical design for laser diodes with large aspect ratio,” 49th Electronic Components and Technology Conference, Jun. 2–4, 1999, San Diego, Calif., USA).
Method 4—Cylindrical Lensed Fiber Method
A cylindrical lensed fiber (CLF) has also been used. (Soon Jang “Automation manufacturing systems technology for opto-electronic device packaging” 50th Electronic Components and Technology Conference, May 21–24, 2000, Las Vegas, Nev., USA). Although the coupling efficiency with the use of a CLF can be high (.about. 90%), the cost is also high because a CLF is not easy to make, and achieving high coupling efficiency also requires difficult labour-intensive alignment, as a practical matter.
Method 5—Laterally Tapered Rectangular Waveguide on Top of a Large Rectangular Waveguide Method
Another approach to mode-size conversion is to place a laterally tapered rectangular waveguide on a large mode size rectangular waveguide, where light coupling between the top and the bottom waveguide occurs as a result of the top lateral taper. This method can serve the function of mode-size conversion in both the vertical and horizontal directions, but it is not well accepted in practice due to the difficulty in integrating such a structure with a device waveguide and also the cost of manufacturing such a structure. FIG. 6 shows such a polymer based waveguide structure 610 inserted between a semiconductor laser 620 and a fiber 630. (D. J. Goodwill et al., “Polymer tapered waveguides and flip-chip solder bonding as compatible technologies for efficient OEIC coupling,” 47th Electronic Components and Technology Conference (ECTC), May 18–21, 1997, San Jose, Calif., USA). One difficulty in this approach is the integration of such a tapered waveguide 610 made of polymer with a laser 620 made of semiconductor material due to the large difference in their coefficients of thermal expansion and mechanical stabilities. In the case where such a structure is made of the same semiconductor material as that of the semiconductor laser, it would require the epitaxial growth of a large bottom waveguide layer and the cost will be high.
Method 6—Vertically Tapered Down Rectangular Waveguide Method
To enable easy integration, vertically tapered down semiconductor waveguide spot-size converters that squeeze the guided optical mode into the cladding have been integrated with semiconductor lasers. (Aaron E. Bond et al., “High speed packaged electroabsorption modulators for optical communications” 50th Electronic Components and Technology Conference, May 21–24, 2000, Las Vegas, Nev., USA; Y. Inaba et al., “Multiquantum-well lasers with tapered active stripe for direct coupling to single mode fiber” IEEE Photonics Technology Letters, Vol. 6, pp. 722, 1997; M. Kitamura, “Method of making a tapered thickness waveguide integrated semiconductor laser,” U.S. Pat. No. 5,792,674, issued Aug. 11, 1998; Jeon et al., “Laser diode device having a substantially circular light output beam and a method of forming a tapered section in a semiconductor device to provide for a reproducible mode profile of the output beam,” U.S. Pat. No. 6,052,397, issued Apr. 18, 2000). Although this method can enlarge the optical mode in the vertical direction, Problems associated with such structures include the required length of the tapered down structure that will lead to additional light propagation loss and the additional expense of III–V semiconductor materials.
2) Costs of Photonic Device Module Connection with Optical Fibers [Problem #2]
While the above-mentioned methods may be employed to transfer optical energy somewhat efficiently between an optical fiber and a device waveguide of about 2 μm in size, the approaches of these methods are costly. Typically, an enclosure is used to house the device, the discrete mode-transferring element (e.g. a ball lens), and the optical fiber, thereby forming a packaged module. To align the device waveguide to the fiber and the mode transferring module, most photonic device manufacturers are still performing manual alignment under a microscope because of the very disparate nature of the components, their high price and low product volumes. Such a process is not well suited to high-volume, low-cost production.
Existing techniques for fixing a fiber (and lens) in position with respect to a rectangular semiconductor waveguide include epoxy curing, soldering, mechanical fixture, and laser welding. In order to reduce the need for manual placement/alignment and fixing in the packaging process, efforts have been focused on automating the fixing process. For example, Newport, JDS-Uniphase and NEC are developing automatic parts-handling and assembling procedures using machine vision combined with micro-stages or micro-robots to achieve sub-micron precision (Soon Jang, “Automation manufacturing systems technology for opto-electronic device packaging,” 50th Electronic Components and Technology Conference, May 21–24, 2000, Las Vegas, Nev., USA; Peter Mueller and Bernd Valk, “Automated fiber attachment for 980 nm pump module,” 50th Electronic Components and Technology Conference, May 21–24, 2000, Las Vegas, Nev., USA; Kazuhiko Kurata, “Mass production techniques for optical modules,” 48th Electronic Components and Technology Conference, May 27–28, 1998, Seattle, Wash., USA).
At the same time, the concept of a silicon optical bench (SiOB) on which V-grooves are wet-etched to guide the mounting or placement of photonic components including fibers, lenses, and even semiconductor chips has been well accepted; SiOBs are disclosed in several U.S. patents (e.g., Murphy, “Fiber-waveguide self alignment coupler,” U.S. Pat. No. 4,639,074, issued Jan. 27, 1987; Albares et al,. “Optical fiber-to-channel waveguide coupler,” U.S. Pat. No. 4,930,854, issued Jun. 5, 1990; Benzoni et al., “Single in-line optical package,” U.S. Pat. No. 5,337,398, issued Aug. 9, 1994; Francis et al., “Waveguide coupler,” U.S. Pat. No. 5,552,092, issued Sep. 3, 1996; Harpin et al., “Assembly of an optical component and an optical waveguide,” U.S. Pat. No. 5,881,190, issued Mar. 9, 1999; Roff, “Package for an optoelectronic device,” U.S. Pat. No. 5,937,124, issued Aug. 10, 1999). It is very likely that high precision automation will be combined with silicon V-groove technology to produce fiber-pigtailed or fiber-connectable photonic devices. The V-groove technology, however, still needs some alignment procedure.
In spite of the above-mentioned approaches, the current packaging cost is still very high. For example, about 70 to 80% of the total cost of any fiber-pigtailed III–V optoelectronic module such as an optical transceiver is due to its packaging. Moreover, most of the prior art has been aimed at solving the semiconductor-laser-to-fiber coupling problem, which is one-directional. For future dense wavelength division multiplexing (DWDM) optical communication systems, bidirectional multi-port devices like M×N switches will be in large demand, and the prior art is not able to provide an adequate solution.
(3) Difficulty of Current Methods for Multiple Fiber Connections [Problem #3]
The current methods are somewhat adequate for large photonic device with one input/output waveguide, they generally become difficult when more than one input/output and fibers are involved. This is because the yield for the alignment procedures referred to above rapidly decreases as the number of input and output fibers increases. This yield reduction can seriously limit the application of such coupling and packaging techniques to high-density photonic integrated circuits, for which tens to hundreds of input and output fibers are expected to be connected to a single photonic chip.
The main criteria needed for optical mode transferring methods and devices to achieve a cost-effective and efficient optical energy transfer between a device waveguide and one or more optical fibers can be more specifically described as follows:                (i) The methods and devices should be able to achieve mode size transformation from about 10 μm down to about 2 μm (for λ=1.55 μm) or in the reverse direction for typical waveguide devices.        (ii) The methods and devices should be able to achieve mode size transfer from about 10 μm to below 1 μm (for λ=1.55 μm) or in the reverse direction for more challenging waveguide devices such as high-density semiconductor photonic integrated circuit.        (iii) The methods and devices should be capable of achieving self-alignment between the photonic device and the optical fibers or other intermediate beam transferring elements. Self-alignment lends itself to low-cost manufacturing. It also allows cost-effective coupling between a photonic device and more than one optical fibers.        (iv) The methods and devices should have low optical reflection and absorption losses between the photonic device and the optical fibers.        (v) The methods and devices should have the flexibility of transferring the vertical and lateral mode size separately. This allows it to correct for beam astigmatism in the device waveguide mode.        (vi) The methods and devices should have high yield and low fabrication costs.        
The current mode transformation methods can not adequately achieve the majority of criteria (i)–(vi). For example, the ball lens method can achieve (i) and (iv) but not (ii), (iii), (v) and (vi)
What is still needed in the field, therefore, are devices and methods for transferring the mode size between photonic device and one or more optical fibers that satisfy some or a majority of criteria (i) to (vi) above.
The present invention described herein overcomes the various difficulties encountered by the previous methods by the use of new optical structures referred to as integrated composite coupling structures (ICCS). The mode transformation device or mode transformer is referred to as an Integrated Composite Mode Transformer (ICMT). With the use of the new optical structures according to the present invention, disadvantages associated with prior methods are addressed.